The chemical composition of surface films formed on air exposed bromine/methanol etched InP were investigated in both the ’’as etched’’ condition and after in situ heat treatment. Both UPS and XPS spectra were obtained in order to probe at two different depths into the surface (≊5 and 15 Å, respectively). The as etched surface appears to be composed primarily of In₂O₃ with a small concentration of a phosphate compound, presumably either InPO₄ or H₃PO₄. Prolonged exposure to water or water vapor causes additional film growth. The new growth increases the phosphate concentration and an In compound which, by the location the In3d₅/₂ peak at 445.6 eV, indicates either InPO₄ or In(OH)₃. Heating the as etched surface in situ also causes film growth. The phosphate is found to evaporate from the outer surface of the film but to grow underneath the In₂O₃ skin as the substrate is heated. It is suspected that excessive H₂O on CO background in the vacuum chamber caused a rapid oxidation of the surface as the sample was heated.